Constanta Dascalu

Controlling the orientation of microgrooves and the depth of the ripple structure in dye-doped liquid crystal cells

The permanent structure induced by an Ar+ linearly polarised laser beam on an asymmetric dye-doped nematic liquid crystal cell was investigated in a pump-probe experiment. The polarisation direction of the probe beam was parallel to the easy axis and two configurations of pump beam were used: polarisation directions perpendicular and parallel to the easy axis. The transmitted intensity of the probe beam was recorded during irradiation and it was observed that it depended both on the power and polarisation direction of the pump beam. We explained the evolution of the transmitted intensity and evaluated the start time of formation of the ripple structure. The induced permanent structure in the irradiated zones and the laser-induced surface morphology was studied using a polarising optical microscope and an atomic force microscope, respectively. The surface morphology in the irradiated zones was also dependent on both the power and polarisation of the Ar+ laser beam. The orientation of the microgrooves in the ripple structure was parallel to the polarisation direction of the pump laser beam in both configurations. For a given pump power, the depth of the ripple structure was greater in the case of an Ar+ beam polarised parallel to the easy axis. The induced azimuthal anchoring energy provided by the ripple structure was evaluated.

Surface morphology of doped nematic liquid crystals: role of dye concentration

We report on laser-induced ripple structures in dye-doped nematic liquid crystals for different dye concentrations varying between 0.5% and 2% wt. The surface morphology was studied by atomic force microscopy. For dye concentrations smaller than 0.56% the ripple structure was not obtained. The depth of the ripple structure depends on both the laser power and the dye concentration. Using the Berreman formula for periodic structure, the azimuthal anchoring energy was evaluated. An increase of the anchoring energy with the dye concentration and with the pump power was noted.

The influence of the easy axis on laser-induced ripple structures in dye-doped liquid crystal films

We have investigated the role of the easy axis in the photoalignment effect induced by an Ar+ linearly polarized laser beam on a dye-doped nematic liquid crystal cell in a pump–probe experiment. The induced permanent structure in the irradiated zones was studied using a polarizing optical microscope, and the laser-induced surface morphology was studied by atomic force microscopy. The depth of the ripple structure depends on both the pump power and the irradiated surfaces. The azimuthal anchoring energy provided by the ripple structure was evaluated and it was concluded that it varies with the cells symmetry.

Role of dopants on the electro-optic effect in nematic liquid crystals aligned with doped polypyrrole

In order to estimate electro-optical response times, measurements of the transmitted light intensity through positive uniaxial nematic thin films in planar alignment have been performed. Very fast relaxation times were experimentally found for nematic samples aligned with conductive polymers, namely, polypyrrole doped with 5 types of anions having different molecular sizes and geometries. Using undoped polypyrrole alignment layers, slower electro-optical responses were obtained. Given that the improvement in the relaxation times is induced by the presence of the anions in the polypyrrole films, the increasing of the total restoring torque of the nematic molecules to the confining surfaces—responsible for the short relaxation times—is related to the charges accumulated at the nematic-conductive polymer interface.

A Percolation Model to Evaluate the Correlation Length of Dye-Nematic Liquid Crystal Interaction

ABSTRACT The orientation effect of methyl-red (MR) molecules on pentylcyanobiphenyl (5CB) nematic liquid crystal has been investigated in a pump-probe experiment at various concentrations of azo-dye and for different power values of an exciting Ar+ laser beam. Excited dye molecules reorient the LC molecules within their correlation spheres. There is a percolation probability that the whole LC bulk be reoriented. Analyzing the threshold energy necessary for this percolation in terms of dye concentration, the model allows evaluating the lowest concentration of dye molecules (critical concentration), the correlation length and the quantum efficiency of the reorientation process.

Nd:YAG laser continuous wave-pumped Q-switched by hybrid passive-active methods

The output characteristics of a continuous-wave flash lamp pumped Nd:YAG laser, passively Q-switched by Cr4+:YAG are reported. In order to control the small-signal transmission of the passive Q-switch, a new method, which made use of a miniature laser placed out of the resonator, is described theoretically and proved experimentally. Further, the combination of a Cr4+:YAG passive Q-switch and an acoustooptic Q- switch that work together in a Nd:YAG CW pumped laser was investigated. With a two-rod resonator and a Cr4+:YAG crystal as saturable absorber, an average laser output power over 180W was reached. The pulse-to-pulse stability was 1%. Folded resonator was used in order to obtain intracavity second harmonic generation in active and passive Q-switch mode.

Electro-optic response of some chiral liquid crystals with high spontaneous polarization

The electrooptical properties of some SSFLC devices with high spontaneous polarization liquid crystals are presented. The switching time was determined as function of electric field strength and a.c. frequency. Using the switching time the rotational viscosity (gamma)

Magneto-optical behavior of ferronematics

curly phi was determined. The results are explained by considering the spectacular molecular relaxation, known as Goldstone mode. The surface stabilized ferroelectric liquid crystal (SSFLC) devices offer memory (bistable) capability, fast response speed and sharp threshold and have a variety of applications in electronics.

Electro‐optic properties of a new ferroelectric liquid crystal with isotropic‐chiral smectic‐C phase sequence

The behavior under magnetic fields of new types of ferronematics is examined. First, by using both optical and electrical method, an increase in the critical field for Freedericksz transition was noticed in cells containing ferronematics. Second we give an explanation of the relaxation processes occurring in ferronematics when the magnetic field was suddenly varied. When ferronematics are involved, the relaxation phenomena are described by lower time constants when compared to those of pure nematics.

Thickness dependence of the anchoring strength in dye-doped nematic liquid crystal cells

The electro-optic properties of a new ferroelectric liquid crystal that shows isotropic (I)-chiral smectic-C (SmC*) phase transition are studied. This exhibits bistable electro-optic switching in the SmC* phase. The dependence of the rotational viscosity versus temperature are determined.